This invention relates generally to seismic exploration of substrata beneath bodies of water and, more particularly, to marine seismic cable used to sense compressional waves (p-waves) reflected from such substrata and/or to sense shear waves (s-waves) reflected from such substrata in response to downwardly traveling compressional or shear waves.
In very simplified terms when traveling within the earth, the seismic waves, whether generated by a compressional wave source or a shear wave source, interact with the earth media. Such interactions include reflection, transmission, refraction, and mode conversion at interfaces between materials (layers) with different densities or velocities.
The direction of particle motion caused by the seismic waves determines the type of wave produce in the earth. The downward propagating compressional waves generate reflected p-waves and converted s-waves. The downward propagating shear waves generate s-waves and converted p-waves.
The p-wave is defined to be a wave which induces particle motion back and forth in line with the direction of the wave propagation. The s-waves are defined as waves for which particle motion is back and forth transverse to the direction of the propagation. There are two independent s-waves components, which are herein called S.sub.1 and S.sub.2. S.sub.1 and S.sub.2 are perpendicular to each other.
The path of the p, S.sub.1, and S.sub.2 propagation could be any direction; however, since vector particle motions are involved, motion detectors aligned with the axes of an orthogonal coordinate system can be used to detect the p-waves and the s-waves within the earth. This is possible because any 3-dimensional vectorial wave field can be represented by 3 independent vectorial components which correspond to the alignment of the motion detectors. For the case of acquisition along a line, these vectorial components are defined herein to be a vertical component and two horizontal components. The two horizontal vectorial components are an inline component and a crossline component which is perpendicular to the inline component. Detectors along the vertical, inline, and crossline directions will sense individual vectorial components of the particle motion in these directions, respectively. The collected data will then have to be processed to separate it into the p, S.sub.1, and S.sub.2 waves.
At the bottom interface, i.e., the bottom of the sea, we have a transition from earth to fluid; an s-wave is unable to propagate in a fluid; therefore only a p-wave (compressional wave) will continue beyond this point into the fluid (the sea or other body of water). The p-wave generates a pressure field in the water which can be detected by a pressure detector, e.g., a hydrophone.
Marine seismic exploration is generally conducted by towing a seismic streamer cable at a given depth through the ocean or other body of water. The streamer is provided with a plurality of pressure sensors, such as hydrophones, disposed at appropriate intervals along its length. The pressure sensors (hydrophones) detect the p-waves produced in the water by the reflected seismic waves and provide electrical signals indicative thereof to suitable recording and processing equipment located on the seismic vessel that is towing the streamer.
In some locations due to the congestion of surface obstacles and the length of streamer cable, it is not possible to use streamer cable to collect p-wave data. Bottom cable containing pressure sensors can be laid on the bottom (the upper most surface the strata) to collect these data.
S-waves contain additional information of the nature of the substrata. However, the detection of shear waves is not practical by towing a streamer. As mentioned, shear waves do not propagate through a fluid medium, nor do pressure sensors sense particle movement.
S-waves can be detected at the bottom interface. The best means for the detection the s-waves at the bottom interface is by the placement of motion detectors on the bottom (the uppermost surface of the strata). The detectors can be aligned with the axis of an orthogonal coordinate system, such as the vertical, inline and crossline axis, although other coordinate systems are possible.
Bottom seismic cable with a plurality of motion detectors, such as geophones, disposed at appropriate intervals along its length have been used in the past to detect shear waves; for example, see U.S. Pat. Nos. 4,942,557, Marine Seismic System by A. J. Seriff, 4,725,990, Marine Shear Cable by A. M. Zibilich, and 4,870,625, Marine-Shear-Wave Detection System Using Single-Mode Reflection Boundary Conversion Technique by D. R. Young and R. C. Swenson. However, these detection methods do not obtain the completely general set of seismic wave measurements described, i.e., measurements of the pressure field and vertical, inline and crossline motion components using separate detectors within a bottom seismic cable. If these measurements were available, they would allow considerably improved data processing to determine p, S.sub.1, and S.sub.2 seismic waves components.
Bottom cable and streamer cable have similar construction and suffer from some of the same limitations. Both types of marine seismic cables have a protective cover (e.g., a polyurethane tube), connecting couplers, stress members and spacers. Both contain electrical components: Streamer seismic cable has detectors which typically include pressure transducers. Bottom seismic cable can contain detectors which typically include either or both pressure transducers and motion detectors. Each type of marine cable has wiring to connect to the electrical components at their termination points. Uninsulated terminations will short out if in contact with sea water; consequently an essentially nonconductive fluid (e.g., a hydrocarbon oil such as kerosene) is used in both cable types to provide insulation and prevent the entry of seawater. A section of marine seismic cable can be as much as 4 inches in diameter and over three hundred feet long, and when joined with other cable sections, the marine cable can be 4 miles or more.
The current trend in marine seismic data collection is to require seismic cables of longer lengths than were required in the past. In addition there is a need for a more robust bottom seismic cable that can operate at greater depths.
To make marine cable longer, additional seismic cable sections are added to its length. A seismic cable must be able to sustain tension forces along its length. Stress members within each seismic cable section absorb these tension loads and transfer these tension loads to the next cable.section, If they are to be interchangeable, they must be able to sustain greater tension forces. For example, as exploration depth increases, a bottom cable must be able to support the weight of the seismic cable extending below it. If the weight of the marine cable suspended from the ship becomes greater than the tension forces the cable section can sustain, the cable section will break. A streamer cable has a density close to water, so its weight in water will not be a significant factor, but as the length increases the drag forces on the cable will increase, increasing the tension forces on the cable.
The need for the seismic cable section to sustain increased tension forces requires not just finding stronger material for the stress members and the couplers because the increased tension exerted on the seismic cable section has other effects. For instant, when a seismic cable section is reeled in for storage or repair, the tension exerted on the cable at the storage reel can be enough to crush the seismic cable section.
At the cable storage reel, the tension exerted on marine cable as it is stored on the cable reel is a combination of several factors: the dry weight of cable suspended out of water; the water weight of cable which is subject to the pulling force of the cable storage reel; the drag forces on the cable; and the rocking motion of the ship as it responds to waves. All of these forces, which act longitudinally on the cable, are also transferred to a transverse force which squeezes the seismic cable section as it goes on the cable storage reel. As the cable is wound on the storage reel, the longitudinal tension force remains constant, but the transverse force becomes a cumulative transverse force on the cable, i.e., the squeezing force continues to build up as additional layers of cable are wound on top of it. The surface of a conventional oil filled seismic cable section cannot sustain a substantial lateral force. The cumulative lateral force could compress the oil filled cable so that oil seals on the cable are broken. If the forces are great enough, instrumentation within the cable can be crushed or the outer layer could rupture.
As the seismic cable section is taken up on a storage reel, since the longitudinal force remains constant, the cable is also subject to internal forces which can damage the integrity of the seismic cable section because of the unequal bending radius between the top and bottom of the seismic cable section. When the seismic cable section is bent over the cable storage reel, the bottom portion of the seismic cable section is subject to internal compression forces and the top portion is subject to internal tension forces. Wires, detectors, terminations, integrated cables, spacers, connecting couplers and stress members are subject to movement due to these forces and could break or deform. A particular problem is the movement of spacers. Current practice is to firmly attach the spacers to stress members so that they will not move under this stress. The attachment of spacers to the stress member is a costly and time consuming task in the construction of the seismic cable section and does limit the type of material that can be used as spacers because the spacers must also be able to sustain the transverse force on the cable.
Oil or some other hydrocarbon such as kerosene has a lower density than water; this is an advantage when used with streamer cable because of the additional buoyancy it offers to cable. However, an oil filled bottom cable will not completely couple to the bottom along its entire length unless additional weights are distributed uniformly throughout the cable length. This has not been practical to do in the past; consequently buoyant sections of the bottom seismic cable section can substantially reduce the effect of the coupling, i.e., the detector and the earth move jointly, of weighed sections of the cable to the bottom. Although inadequate coupling between the bottom cable and the bottom will have little effect on the upward traveling p-waves which can be detected by a pressure sensor in a fluid medium, it is of critical importance to have adequate coupling to the bottom for the detection of the particle motions. Each motion detector must be coupled to the bottom so that all components of the upward traveling vectorial wave field, i.e., the three dimensional components of the p-wave and s-waves, can be detected so that specific values of s-waves can be determined through processing of the seismic data.
There is an another effect which buoyant sections have on an oil filled bottom cable. When a bottom cable is dragged into place, tension is placed on the cable. If the density of each part of the cable is greater than the density of water, each part of the cable will be in contact with the bottom. The dragging motion will remove kinks from the cable and straighten out the cable as it is moved. When the cable comes to rest, because all of the cable is in contact with the bottom, the friction exerted between the bottom and the bottom cable will be enough to maintain tension in the cable so that it will remain straight. Specific detectors installed within the marine cable along its length can then be particularly located when wave forms sensed by those detectors are processed. If parts of the bottom cable are buoyant there may not be enough frictional forces due to insufficient bottom contact to keep kinks out of the cable or to retain the cable in a straight line after the cable comes to rest. The ability to correlate collected wave forms with a specific detector location will be impaired.
Oil filled bottom cable, because it is dragged into place for some operations, is not robust. Rocks or other debris on the bottom could rupture the cable allowing the oil to leave the cable and the water to enter shorting out the electrical connections.